Nickel electroplating operations often play an important role in semiconductor and integrated circuit fabrication processes. For instance, in a typical wafer level packaging (WLP) application, the formation of a “bump stack” may involve the electrodeposition of a relatively thin nickel layer (1-5 μm) to serve as a copper diffusion barrier between a copper seed layer or copper pillar and a solder layer formed of tin or tin-silver material. In the absence of such a diffusion barrier, the copper reacts with the solder layer and forms a very thick and weak intermetallic layer. FIG. 1A, for example, displays a cross-sectional view of the interface between a copper seed layer and a solder layer of tin-silver deposited directly thereon, and illustrates the formation of a thick intermetallic layer between the deposited solder layer and the underlying copper seed in the absence of an intervening nickel barrier layer. FIG. 1B shows a similar cross-sectional view of copper seed and solder layers but here with an intervening nickel barrier layer. FIG. 1B illustrates, in contrast to FIG. 1A, that the intermetallic layer potentially formed—here between the nickel barrier layer and the tin-silver solder layer—may advantageously be quite thin. FIG. 1A also points out several so-called “Kirkendall Voids” which oftentimes form within the thick copper/tin-silver intermetallic layer in the absence of an intervening nickel barrier layer.
In semiconductor fabrication processes, nickel electroplating is frequently performed using a nickel sulfamate-based electrolytic bath—in particular, in advanced nickel plating applications where low stress films are a requirement (such as WLP). Nickel sulfamate baths are composed of dissolved nickel sulfamate salts typically in combination with boric acid and an “anode activator” ingredient. Several commercially-available formulations will be discussed in greater detail below. Typically, the target acidity of these baths is within a pH range broadly of about 3.0 to about 5.0, and sometimes within a more limited range of 3.5 to 4.5.
Nickel sulfamate electrolytic baths are typically employed because of nickel's high solubility in sulphamic acid—meaning that a higher concentrations of dissolved nickel ions are possible than with other nickel electrolyte solutions—which can result in higher electroplating rates than may be achieved with other potential nickel electroplating solutions. In addition, nickel sulfamate electrolyte solutions are able to produce very low-stress electrodeposited films.
Nevertheless, despite these clear advantages, basic nickel sulfamate electroplating solutions (and even those containing boric acid plus an “anode activator”) still fail to produce ideal films of electroplated nickel without some additional engineering of the electroplating chemistries. Chief among the remaining issues is the surface roughness of the electrodeposited film, which has been found to be associated with the formation of detrimental interfacial voids between nickel film and reflowed solder material. For instance, FIG. 1C presents two electron micrograph images showing the surface roughness of two electrodeposited nickel films and the tendency of this “roughness” to cause wafer defects. As shown in the figure, an electrodeposited nickel film having a surface roughness (Ra) of 35.6 nm is seen to result in a wafer exhibiting a large defect count relative to a wafer having an electrodeposited nickel film with a surface roughness of roughly half that at 14.2 nm.
Moreover, in a typical nickel electroplating process flow, such as that employed in a typical WLP application, multiple nickel sulfamate baths are used to sequentially plate multiple semiconductor wafers. Since deviations in bath composition can also result in inferior electroplating, poor process performance, and potential defects in the plated nickel layers, ideally, each semiconductor wafer is plated under substantially the same process conditions, relatively invariant with time and constant over the plating of numerous wafers. In practice, however, maintaining constant process conditions in nickel sulfamate baths can pose a significant challenge.